Can knee braces be considered as a brace point for top flange of through girder bridges?

[rsbmusicguy]

Hey all,

See my question above, a colleague and I are going back and forth on the issue. Note my bridge is a through girder bridge with the deck of the bridge lying below the neutral axis of the through girder. Although knee braces are present at the inside face of the girder, I am arguing that the knee braces should not be considered as a brace point for the top flange and only be taken as transverse stiffeners for the through girder.

I found a good paper on the subject, read through and looks like they concluded (on Page 11 of 13):

"The system is not a true braced condition, it more like a system of connected frames. The top flanges of the TPGs are relatively free to rotate with no true means to restrain the out of plane distortion (e.g. struts, cross bracing)."

Can anyone argue this or have more experience on the subject?

Thanks,

RSB

 

[wiktor]

The knee braces when framed into floorbeams are essentially half frames and typically are considered as restrain points preventing top flange buckling.
More on this subject in the attached article.

 

[rsbmusicguy]

Wiktor,

I appreciate your reply and forgot to reference this paper in my initial post.

Note on Page 11 of 13 under conclusions it reads : "The system is not a true braced condition, it more like a system of connected frames. The top flanges of the TPGs are relatively free to rotate with no true means to restrain the out of plane distortion (e.g. struts, cross bracing)."

This leads me to believe that their findings show that knee braces should not be considered as brace points for the top flange? This was a surprise to me...

Thanks again!

RSB

 

[wiktor]

RSB,
I have some old (1966) text book called "Metal Bridges" unfortunately not in English. The figures and formulas are exactly the same like presented in the paper.
The braces cannot be considered as stiff brace points, but rather as horizontal spring supports, preventing buckling of the flange and web.
However, the paper does not conclude that the knee braces should not be considered - only that these should be analysed carefully. Modeling of the span in any of FEM capable software will be perhaps an overkill, so I will rather stick to AREMA formulas.

 

[rsbmusicguy]

Great - thanks for your advice!

RSB